The Mekong floodplains and delta play an important role in poverty and hunger alleviation in Vietnam and Cambodia. Their high agricultural and ecological productivity are largely due to the natural hydrological regime and rich sediment of the Mekong River. However, regional demand for electricity is leading to the development of over 120 upstream hydropower dams, which may alter the hydrology of the floodplains and delta. Within the floodplains, extensive delta-based flood protection systems, in the form of dykes, are being constructed to increase agricultural production in certain parts of the delta, but which can have negative effects on other parts of the delta. Rising sea levels due to climate change and ongoing land subsidence will exasperate flooding in unprotected regions. The main aim of this study is, thus, to investigate the effect of water infrastructure development (both dams and dykes), sea level rise and land subsidence on the floodplains‟ hydrology and sedimentation. This goal was implemented by three methods: historical data analysis, remote sensing data analysis and numerical modelling.

In terms of hydrology, measured data analysis showed that the impact of hydropower dams was currently limited to the upper part of the Mekong floodplains (the Cambodian Lowlands). Flood prevention in the upper Vietnamese delta is the main driver of hydrological regime alterations. Dykes significantly reduce flooding areas and increase rising and falling rates of water levels in the middle floodplain. In the lower part of the floodplains (the middle Vietnamese delta), hydrology is not only influenced by the downstream movement of water due to upstream flood prevention systems but also sea level rise and land subsidence.

Results from modelling water infrastructure development, sea level rise and land subsidence scenarios indicated that the effect of each challenge on hydrology is dependent on characteristics of each region. In the future, full development of hydropower dams will increase dry season water levels by 23%, but wet season water levels will only change by slightly over 1% in the upper floodplains (river-dominated region). Flood prevention systems will significantly change water storage capacity and water transfer capacity in the floodplains, causing substantial regional changes in flood patterns. Sea level rise and land subsidence will result in the inundation of a vast region of the Vietnamese coast (tidal region).

In terms of sedimentation, remote sensing data analysis suggested that flood protection systems reduced flooding areas over the period from 2007 to now, and high dykes – a component of flood prevention systems - likely disconnect the protected areas with the rest of the floodplains and prevent sediment from moving into fields. Although semi-dykes reduced the amount of sediment deposition in the rising stage of the flood season, they had no effect on sediment settlement in later stages.

Two-dimentional hydrodynamic modelling proved that all the mentioned challenges would influence negatively on sediment deposition in the floodplains. The development of hydropower dams would cause significant reduction in sediment concentration in water, resulting in the declining of sediment settlement throughout the floodplains. Water infrastructure development would propagete flooding and also shift sediment deposition downstream and neighbouring regions. Sea level rise and land subsidence would increase tidal dynamics in the inland delta; thus, more sediment would be washed out to the ocean.

In the long term, sediment starvation is likely to translate into lower agricultural production; consequently, farmers will have to employ more artificial fertilizers which increase the risk of environmental pollution.

Regional wide transboundary water resource use policies are needed to address future changes in the balance among agricultural productivity, energy generation and the natural environment. More studies on operational optimization to maximize protection areas and the amount of sediment moving into fields are needed to ensure the sustainable development of the region.